%  - - - - - - - - -
%   i a u G s t 0 6
%  - - - - - - - - -
%
%  Greenwich apparent sidereal time, IAU 2006, given the NPB matrix.
%
%  This function is part of the International Astronomical Union's
%  SOFA (Standards Of Fundamental Astronomy) software collection.
%
%  Status:  support function.
%
%  Given:
%     uta,utb          UT1 as a 2-part Julian Date (Notes 1,2)
%     tta,ttb          TT as a 2-part Julian Date (Notes 1,2)
%     rnpb             nutation x precession x bias matrix
%
%  Returned (function value):
%                      Greenwich apparent sidereal time (radians)
%
%  Notes:
%  1) The UT1 and TT dates uta+utb and tta+ttb respectively, are both
%     Julian Dates, apportioned in any convenient way between the
%     argument pairs.  For example, JD=2450123.7 could be expressed in
%     any of these ways, among others:
%
%            Part A        Part B
%
%         2450123.7           0.0       (JD method)
%         2451545.0       -1421.3       (J2000 method)
%         2400000.5       50123.2       (MJD method)
%         2450123.5           0.2       (date & time method)
%
%     The JD method is the most natural and convenient to use in
%     cases where the loss of several decimal digits of resolution
%     is acceptable (in the case of UT;  the TT is not at all critical
%     in this respect).  The J2000 and MJD methods are good compromises
%     between resolution and convenience.  For UT, the date & time
%     method is best matched to the algorithm that is used by the Earth
%     rotation angle function, called internally:  maximum precision is
%     delivered when the uta argument is for 0hrs UT1 on the day in
%     question and the utb argument lies in the range 0 to 1, or vice
%     versa.
%
%  2) Both UT1 and TT are required, UT1 to predict the Earth rotation
%     and TT to predict the effects of precession-nutation.  If UT1 is
%     used for both purposes, errors of order 100 microarcseconds
%     result.
%
%  3) Although the function uses the IAU 2006 series for s+XY/2, it is
%     otherwise independent of the precession-nutation model and can in
%     practice be used with any equinox-based NPB matrix.
%
%  4) The result is returned in the range 0 to 2pi.
%
%  Called:
%     iauBpn2xy    extract CIP X,Y coordinates from NPB matrix
%     iauS06       the CIO locator s, given X,Y, IAU 2006
%     iauAnp       normalize angle into range 0 to 2pi
%     iauEra00     Earth rotation angle, IAU 2000
%     iauEors      equation of the origins, given NPB matrix and s
%
%  Reference:
%     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
%
%  This revision:  2008 May 24
%
%  SOFA release 2012-03-01
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function gst = iauGst06(uta, utb, tta, ttb, rnpb)

% Extract CIP coordinates.
[x, y] = iauBpn2xy(rnpb);

% The CIO locator, s.
s = iauS06(tta, ttb, x, y);

% Greenwich apparent sidereal time.
era = iauEra00(uta, utb);
eors = iauEors(rnpb, s);
gst = iauAnp(era - eors);

